The concept of professional revolutionaries, alternatively called cadre, is in origin a Leninist concept used to describe a body of devoted communists who spend the majority of their free time organizing their party toward a mass revolutionary party capable of leading a workers' revolution. The size of this core is naturally proportional to the size of the party itself.
Most Marxists agree that a cadre is necessary in one form or another; Trotsky in particular did not believe that these professional revolutionaries, or vanguardism in general, was to blame for what they considered the eventual "totalitarian" (what they term Stalinist) nature of the Soviet Union, nor the situations in China, the DPRK, and other Communist states; instead they cite as the true causes the isolation of the Russian Revolution in the case of Russia, the practices of Maoists, and other Marxist-Leninist parties.
Other communists, particularly those sometimes dubbed "post-Maoists", do indeed disagree with the professional revolutionaries concept and view it as antithetical to the "mass party" ideal (party physically composed of the masses of people, rather than an elite intellectual core) advocated especially strongly by Mao, although never realized under his leadership in China. Such communists therefore today advocate for a current version of a "mass party" of working class people, thereby in their view maximizing direct participation in the revolution and the subsequent revolutionary government.
In Lenin's original work the purpose of the cadre is to educate the masses and essentially bring the entire population to the level of "professional revolutionaries", but it was not a requirement that the whole population, or even a majority, be at or near such level before seizing power in a communist revolution. At its highest point of membership, in the 1980s, the Communist Party of the Soviet Union contained only 18 million out of a total area population of 280 million, and it is unknown how small fraction of those 18 million could have been regarded "professional revolutionaries".